1. Field of the Invention
The present invention is generally related to Optical Network Terminals (ONTs), and particularly to automatically controlling the ranging of an ONT with an optical signal present downstream of an Optical Line Terminal (OLT). The present invention allows the ONT to automatically disable itself if it fails to range with the optical signal present downstream of the OLT after a number of attempts.
2. Description of the Related Art
A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to an end user. The term “passive” simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network.
A traditional PON consists of an OLT at a communication company's office and a number of Optical Network Terminals (ONTs) near end users. An ONT is a form of Access Node that converts optical signals transmitted via fiber to electrical signals that can be transmitted via coaxial cable or twisted pair of copper wiring to individual subscribers.
The OLT provides the interface between the PON and a number of ONTs typically connected in a star arrangement using optical splitters, which typically reside at a premise of a user. Multiple ONT units share a single upstream wavelength to communicate with the OLT at the head end. The OLT is responsible for allocating upstream bandwidth to the ONTs. Each ONT unit is assigned a specific time slot in which to communicate. ONTs are situated at varying distances from the OLT, meaning that the transmission delay from each ONT is unique. This presents an optical multipoint-to-point bus that is subject to corruption if an ONT unit transmits at any time other than during its assigned time slot or the transmitter is staying on. When this occurs, the ONT may disrupt the PON servicing other ONTs. ONTs are not controllable from the OLT and any untimely transmission from the ONT may result in optical collisions on the shared optical media located between the OLT and the splitter servicing the other ONTs thus resulting in a loss of service.
A further problem arises when an ONT suffers a loss of service during normal operation, due to a maintenance issue, power outage or other equipment/hardware problem. In order to resume the lost service, the ONT attempts to range back to the system by continuously sending handshake signals at random times to the OLT. This can easily result in an optical collision, and disruption of service to other ONTs on the system.
In an effort to overcome these deficiencies, there have been attempts to provide solutions to this problem by introducing various hardware components such as “watchdog timers” which can be reset. Such timers detect the excessive laser use by the ONTs and resets the normal laser enable control signal. If the control signal does not reset the watchdog timer, then the timer times out and inhibits the transmit laser from coming on. Other solutions attempt to detect when an ONT unit becomes disruptive and identify an uncontrolled unit by variation in power levels to the system. These levels are compared to the power levels associated with the normal operation. If the variation is noticed, all the ONTs are turned off and then turned on sequentially by providing and maintaining a time lag so the transmissions do not interfere with each other.
There is no known mechanism to allow ONTs to recognize that they are not synchronized with an allotted time slot. As such, they keep on transmitting and disrupting the transmission to other ONTs in the network. All the known solutions require a positive evidence of malfunction which require a hardware implementation. Such an implementation is costly and the solution that can be provided after such a cost is incurred is undesirable.
Therefore, a more reliable means of detection is required to prevent the disruption of a system by an uncontrolled ONT unit.